1. The Field of the Invention
This invention relates generally to digital communication systems and, particularly, to such systems which provide for multiple access to a plurality of signals carried on a single communications medium. More specifically, this invention relates to telephone extension systems, by which signals are transferred simultaneously between a plurality of telephone lines and telephone extensions by means of the AC power lines of a building or an RF (radio frequency) transmission medium.
2. The Background Art
When conventional telephone systems are installed in a building, a significant expense is frequently associated with running the necessary telephone wires for all desired telephone extensions. In an existing building, the telephone installation process may also significantly disrupt the building's normal use. Moreover, due to the time and expense involved, the installation is very often not susceptible to convenient modification, despite changes in the needs of the telephone system user.
In an effort to overcome the foregoing disadvantages, various types of wireless telephone systems have been developed. Wireless telephone systems typically include a base unit which receives the telephone signal from a conventional telephone line. The signal is then transmitted between the base unit and one or more extension locations in some manner. Most commonly, the telephone signal is transmitted between the base unit and the extensions using conventional radio frequency (RF) transmission signals and techniques. More recently, however, attempts have been made to transmit the telephone signal using the existing power lines of the building. These prior efforts have had varying degrees of success.
For example, one of the major deterrents in transmitting telephone signals over existing power lines is the nature of the power line medium itself, which presents a low and variable impedance to carrier signals as well as an extremely noisy communications environment. Studies have demonstrated that the optimum carrier frequency range lies between 3 and 15 MHz. Most prior art attempts to operate below 2 MHz have failed commercially because of noise or interference problems from other equipment operating on the electrical system.
Numerous prior art signal modulation techniques have also been attempted, primarily employing FM modulation of the carrier by audio (speech) signals (U.S. Pat. Nos. 3,949,172 and 4,701,945 being examples, the disclosures of which are incorporated herein by this reference). The problem with FM modulation is that no security is afforded the users; that is, other users with the same devices can make calls on another user's line and eavesdrop on conversations. The impact of these problems has already been demonstrated in the cordless telephone industry, which shares the same limitations as the line carrier industry. Additionally, commercial AM and FM broadcast stations are often heterodyned and demodulated in the RF range utilized by these systems, thereby interfering with the reception of telephone conversations. The transmissions from one of these systems will often also radiate and interfere with other types of FCC licensed and unlicensed commercial and residential equipment. Even the use of two FM modulation stages, as described in U.S. Pat. No. 4,701,945, is not able to solve these problems.
In prior art systems, full duplex voice communication is usually attempted by using two carrier frequencies, one for each direction. Usually a transmitter and receiver are included in each station which are operating simultaneously. This leads to mutual interference as well as increasing the normal interference drift problems and does not eliminate the security problems.
Recently, attempts have been made to transmit relatively low frequency digital data (&lt;2 Kbs) via a line carrier and employing a multiple access technique known as direct spread. (See, for example, U.S. Pat. Nos. 4,641,322 and 4,864,589, the disclosures of which are incorporated herein by this reference.) Generally, the carrier frequencies (200-500 KHz) and corresponding data rates (20-1000 bs) are too low to provide sufficient processing gain to permit real time full duplex voice communication which generally requires about 100 Kbs. The systems using direct spread techniques also typically employ line carrier remote data collection and control applications for which high speed multiple channel data transmission is not required. Such systems likewise do not typically accommodate more than one system using the same power lines in the same building.
In summary, therefore, no prior art line carrier telephone extension system is known which permits private, multiple line, high quality duplex voice communications which does not interfere with other electronics systems.